Quote:yes this is existing application. I dont have a source code.
Then all you can do is contacting the author of the application.
Because pasting by context menu works, it is probably a bug (I can't see any reason for doing such by intention).
If you can't get it fixed by the author:
Did you tried Shift+Insert?
If you want to use the keyboard only you can also try to use the context menu key (left of the right Ctrl key on many keyboards) and navigate to the paste option using the cursor keys.